The present study has examined the effect of GSH on two lines of IL-2-- dependent activated killer cells, LAK cells and alphaCD3-activated killer (CD3-AK) cells. We found that GSH added during first 24 hour decreased the generation of LAK and CD3-AK cells from resting lymphocytes, whereas after 48 hours of activation, the addition of GSH increased the killer cell activity. In addition, BSO, an inhibitor of GSH biosynthesis, decreased the proliferation and cytotoxic activities of activated killer cells, and the inhibitory effect was reversed by GSH. These results indicate that GSH downregulates the generation of LAK or CD3-AK cells from resting lymphocytes, but it upregulates the further differentiation of preactivated killer cells. The effect of GSH thus varied with the state of activation of the killer cells. Culturing CD3-AK cells in GSH did not change the distribution of T cell subsets, did not affect the cells' ability to produce lymphokine (IL-2), and did not induce suppressor cells. One striking change as revealed by flow cytometry analysis was that the levels of IL-2 receptor and TCR (alpha/beta)-CD3 were reduced by 80 and 30%, respectively, after 48 hour culturing in GSH. Determination of the mRNA of IL-2 receptor suggests that a post- transcriptional block existed. It appears that the negative effect of GSH on the function of surface IL-2 receptors or T cell receptors on resting lymphocytes severely affected the signal transduction through these receptors and thus abrogated or reduced LAK or CD3-AK cell response. In contrast, for preactivated killer cells, upregulation by intracellular GSH of IL-2 utilization is a dominant effect, thus allowing further differentiation of these killer cells. Our results indicate that the balance between the activation signal (IL-2 or alphaCD3) and the immunoregulatory signal (induced by GSH) may determine the outcome of the immune response.